1. Field of the Invention
This invention relates to a transparent conductive film used for a touch panel, especially for an analog type touch panel, method of making the transparent conductive film, a transparent conductive film-coated substrate and a touch panel having the transparent conductive film-coated substrate.
2. Discussion of the Background
Various transparent conductive films for display electrodes, such as liquid crystal displays (LCD) or plasma displays, have been developed. The transparent conductive film for such uses is generally desired to have low electrical resistance.
Sputtering is widely used as an industrial manufacturing method for transparent conductive film having a low electrical resistance. Generally the method has the following steps: placing a transparent insulated substrate and a target made of the desired material of the transparent conductive film in a chamber, and heating the substrate up to at least 200.degree. C. under vacuum, then introducing argon gas and oxygen gas into the chamber and applying a DC negative voltage to the target whereby the transparent conductive film is formed on the substrate. In order to reduce the electrical resistance of the transparent conductive film, the indium oxide target doped with tin oxide within a range of from 5 to 15 weight % (based on In.sub.2 O.sub.3 +SnO.sub.2) is usually used. At the present time, especially, since low electrical resistance of no more than 300 .mu..OMEGA..multidot.cm is required, indium oxide doped with 10 weight % of tin oxide is often used as a standard target for the transparent conductive film.
Films made by this method should be a thin film having a thickness of not more than 10 nm when used for an analog type touch panel, because such panels require a high sheet resistance, such as from about 500 to about 1000 .OMEGA./sq. The definition of .OMEGA./sq is shown in "Thin Film Phenomena, K. L. Chopra, p. 388, 1969, McGraw-Hill". However, such thin films tend to be insufficient in heat resistance or moisture resistance.
As a method of increasing the sheet resistance of a mixed film of indium oxide and tin oxide (hereinafter referred to as an ITO film) produced by sputtering when the film is more than 10 nm thick, the temperature of the substrate can be reduced while sputtering. In that case, however, sufficient heat resistance or moisture resistance cannot be obtained.
JP-A-4-308612 shows that a transparent conductive film which is made of ITO doped with nitrogen is produced by sputtering under an atmosphere containing nitrogen and that the transparent conductive film is suitable for an analog type touch panel from the standpoint of high sheet resistance, high transparency and high moisture resistance. However, the transparent conductive film does not have a sufficiently high transparency required for recent touch panels.